It is known to produce continuously a fleece-like web or mat of thermoplastic synthetic resin filaments by processes which generate continuous filaments, such as the spun bond process described, for example, in German application DE-OS 40 14 413 or from segments of the filamentary product by, for example, the melt blown process described in German Patent DE-PS 40 36 734.
In either case, above a generally horizontal stretch of the sieve belt, forming the receiving table, a source of the filamentary product deposits the filaments continuously on the belt where the web is collected.
The sieve belt itself is generally composed of a woven or knitted wire fabric or wire netting of steel wire, bronze wire or the like. In practice the belt has an elasticity and thus an elongation under the applied tension which is not negligible.
It is important in the long term production of continuous webs as described to maintain a setpoint orientation of the sieve belt with low tolerances so that the orientation is not altered on standstill and start up of the belt and which is maintained with minimum deviations over the duration of web production.
In practice this has been achieved in the past only with highly specialized control devices or controlled rollers or the like over which the endless sieve belt is guided, in addition to the deflecting rollers normally provided at ends of the path of the belt. These earlier control systems have been found to be expensive to set up and maintain, and frequently cannot compensate for temperature gradients across the length and width of the belts and their variations in thermal expansion and contraction and which may detrimentally influence the belt path. When fluctuations in belt path occur, the quality of the product, namely, the homogeneity or consistency of the density distribution in the web can be detrimentally affected.
In other fields in which endless belts of a variety of materials are used and are guided around deflecting rollers at ends of the path, there have been proposals to limit deviations from a desired belt path.
The prevailing thought in these systems is that the longitudinal tension varies across the width thereof In accordance with these teachings, the band has a tendency to move toward the side with the smaller longitudinal tension (cf German published Application DE-AS 26 43 346, col. 2, lines 9-22). With sieve belts and receiving tables for webs of the type described, the prevailing teachings in other fields have not had any significant influence.